This present invention relates to the field of non-destructive testing methods and apparatuses, specifically to non-destructive testing methods and apparatuses to determine metallurgical and physical properties of a ferrous test subject. Iron pipe centrifugally cast from ductile iron will be used as the exemplary test subject throughout this disclosure. Ductile iron is made by treating a low sulfur cast iron with magnesium, which causes the graphite to form spheres or nodules rather than flakes. These nodules give ductile iron its desirable qualities, namely a high modulus of elasticity and, therefore, increased strength. If a certain period of time elapses after the treating with magnesium takes place and before the ductile iron is cast, the magnesium becomes bound up with other elements in the iron and is unable to form nodules. Cast iron in which the graphite forms into flakes is commonly called gray iron. Gray iron has a lower modulus of elasticity. The graphite structure may vary along the length of the pipe, containing both nodules and flakes. It is advantageous for the manufacturer of ductile iron pipe to test the pipe to determine if there is any questionable graphite structure or non-ductile areas along the length of the pipe.
It has been known that the modulus of elasticity in a ductile iron pipe can be determined by measuring the energy of an acoustic wave that has been generated in the pipe wall. The graphite in gray cast iron offers great resistance to the passage of sound waves and, thus, the velocity is lower than that of a sound wave passed through ductile iron. Various methods and apparatuses employing the concept that the velocity of sound waves can be measured to determine the modulus of elasticity in cast iron have been documented and patented.
Diamond, U.S. Pat. No. 3,603,136, discloses a method and apparatus for determining the nodularity of a workpiece as a function of the speed of sound waves through the workpiece. This is achieved by positioning the workpiece at a predetermined distance from an electro-acoustic transducer, or by positioning the casting between two electro-acoustic transducers which are spaced at a predetermined distance. The casting is immersed in water, an ultrasonic pulse is generated by a crystal immersed in the water and has sufficient energy so that it will pass through both the first and second surfaces of the workpiece. Back reflections are produced and the crystal generates signals upon reception of each back reflection. An oscilloscope displays the transmitted pulse and the first and second back reflections, allowing the operator to calculate the total time required for the impulse to travel from the first surface to the second surface. If the crystal is positioned a predetermined distance away from the second surface, the actual thickness of the casting is not required in subsequent calculations which then determine the velocity of sound in the workpiece solely as a function of the time between the display of the pulses.
DiLeo, U.S. Pat. No. 3,844,163, discloses an ultrasonic nondestructive testing system for measuring the velocity at which ultrasonic energy moves through a material. A pair of search units are provided for propagating ultrasonic energy towards the opposite sides of a workpiece and receiving such energy therefrom. A computer measures the various time delays resulting from the ultrasonic energy propagating thorough the workpiece and computes the velocity of the ultrasonic energy in the workpiece.
Bantz, et al., U.S. Pat. No. 3,848,460, also discloses a method of measuring the velocity of sound in a workpiece using transmit and receive ultrasonic transducers spaced a predetermined distance apart from each other in a liquid bath.
In the above references, generation of ultrasonic waves is achieved primarily by some form of electro-mechanical conversion, usually piezoelectricity. The disadvantage of this method of sonic measurement is that it requires a fluid couplant, such as a liquid bath, to mechanically transfer sound generated by the transducer into and out of the workpiece. As the workpiece must be covered with a thin layer of fluid or immersed in liquid, this process complicates testing, making it more expensive and time-consuming.
Buttram, et al., U.S. Pat. No. 5,714,688, discloses a method of examining ductile iron using an electromagnetic acoustic transducer (EMAT) system to determine a time-of-flight of an ultrasonic shear wave pulse transmitted through a casting at a selected location, from which a velocity of sound in the casting can be determined. EMATs are the basis of a non-contact ultrasonic inspection method which requires no fluid couplant because the sound is produced by an electromagnetic acoustic interaction within the material. The method disclosed in Buttram uses first and second EMATs arranged on opposite sides of the casting at a selected location. The thickness of the casting is measured at said location. The first EMAT is energized, thus creating and sending an ultrasonic pulse through the casting to the second EMAT. The pulse is received at the second EMAT and the time required for the pulse to travel through twice the thickness of the casting is measured. The shear wave velocity is calculated using the relationship between the thickness and the measured time value. The degree of nodularity in the casting is determined from a pre-established relationship between the shear wave velocity and the percent of nodularity for ductile cast iron.
Thus, Buttram discloses a method for determining ductility of a workpiece without the need for a fluid couplant, improving on the prior art. However, the method disclosed in Buttram requires a measurement to be made of the thickness of the workpiece at the particular location at which the transmission of the pulse is to occur. As such, a measurement can be cumbersome, time-consuming, and inaccurate in a manufacturing environment, it is desirable to implement a more accurate and efficient method for determining ductility of the workpiece without a requirement that such a thickness measurement be made.
Thus, there is a need for a non-destructive method and apparatus for efficiently and accurately determining the microstructure of ferrous metal objects that is suitable for a modem manufacturing environment, that does not require the workpiece to be immersed in a couplant and does not require the operator to know the thickness of the workpiece being tested.
The present invention provides a non-destructive method and apparatus for determining the microstructure of ferrous metal objects. More specifically, this method and apparatus will allow the user to measure metallurgical and physical properties of a ferrous object such as a cast iron pipe without the need for a couplant or for potentially inaccurate measurements. In the preferred embodiment, the apparatus is composed of a capacitive discharge magnetizer, one or more sensors, signal processing electronics, and a data analysis system. A sonic wave is introduced into the pipe wall by magnetostriction, as disclosed in Watts, et al., U.S. Pat. No. 5,336,998. In the preferred embodiment, the firing of the capacitive discharge magnetizer through a central conductor causes the pipe wall to contract. This contraction produces a sonic wave, which propagates through the pipe wall. Multiple reflections of the sonic wave take place, and the sensor(s) capture the intensity of the sonic waves. After processing through the electronics, the signal is analyzed by the computer software in the data analysis system. The time from the beginning of current discharge to the point at which the displacement of the pipe wall crosses an initial rest point, (the Villari Reversal Point), is measured and compared against a known value to determine ductility of the workpiece.
The present invention has many objects and advantages over the prior art. One such object is to provide a method for determining microstructure of a ferrous metal workpiece quickly and efficiently in a modern manufacturing environment.
A further object of the present invention is to provide a method and apparatus for determining the microstructure of a ferrous metal workpiece without the need for a fluid couplant, thus decreasing the time for testing and reducing the potential for inaccuracies in measurements due to contaminants in the couplant.
Yet another object of the present invention is to provide a method and apparatus for determining the microstructure of a ferrous metal workpiece without the need for a measurement of the thickness of the workpiece, thus minimizing the potential for human error and inaccuracies in the testing process.
Still a further object of the invention is to use the Joule Effect and the Villari Reversal Point in analyzing the processed signal to more easily determine metallurgical properties of ferrous materials.